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Explaining Pres Actions: a Working Document

Author:

David Pearson

Nicholas del Pozo

Publication date:

Thursday, 1 October, 2009

Abstract:

This document attempts to present the different preservation methodologies that are currently available to the library, as well as provide an indication how these methodologies could be used to preserve digital materials. It is the goal of this document to outline the current thinking of the digital preservation branch in such a way that it is possible to decide, in close collaboration with each collecting area, which digital preservation strategies are most appropriate for each collecting area. It is also intended that this should provide a means for identifying where a collection area may require additional resources, as well as where new technologies should be acquired, or developed.

Introduction

This document identifies a number of preservation methodologies that have been identified as building blocks to help build potential strategies for approaching the preservation of digital materials. The methodologies which are examined are:

Migration;

Emulation;

Application based rendering;

Collecting and maintaining a ‘Technology Museum’; and

Taking no action.

For each preservation methodology the following will be presented:

What the methodology is, or what it purports to do;

How it works;

Its perceived advantages and disadvantages; and

Different strategies for approaching or maintaining the methodology.

All these preservation actions presume access to the bit-stream. That is to say, that it is possible to access the physically stored data without being technologically inhibited (e.g., if the digital object is stored on a CD-ROM, it is still possible to read the CD-ROM, and the data stored on the CD-ROM is still intact). Ensuring digital materials are not stored on obsolescent carriers is a significant preservation issue, but is not the subject of this paper. For more thinking on this topic, see Clifton and Langley (2007), Elford et al. (2008), or del Pozo et al. (2009).

This document makes references ideas and terminology which is better defined in the Rethinking Repository Requirements, and Preserving Digital Objects Within the National Library of Australia, two forthcoming documents which, together with this document, make up a suite of technical and theoretical papers addressing various ideas and problems in digital preservation. As is alluded to in these other papers, in order to make a more informed decision about which preservation actions will most adequately preserve a digital object over time, relative to the needs of the preserving organisation, a certain degree of knowledge is required. Ideally, as much as is possible, the following knowledge areas should be known, or there should be some clarity in each of these areas:

The intention of the preserving institution for the object, and an understanding of which aspects of the digital object should be maintained, or which elements of the original digital object are most important to preserve.

The perceived intention of the original creator of the digital object, for how others should experience the material.

Knowledge of how the digital object will be accessed both before and after any preservation action.

Knowledge of the file part of the digital object (where a digital object contains a file part), including the structure of the file format, and the content of the file.

It is proposed in the Preserving Digital Objects Within the NLA document, that a digital object can be thought of as having various aspects, each of which may require a different strategy for long term preservation. These aspects are:

The physical arrangement of the data

The binary sequence of the file derived from the physical arrangement

The information that the binary sequence can be decoded to convey

The interpretation that a user may derive from the information

Each preservation action outlined below can be seen as potentially preserving one or more of these aspects, sometimes at the expense of others. Although it is difficult to generalise in this area, where it has been seen as appropriate, it has been indicated to what degree a given preservation action will require knowledge in these different aspects (data, action and experience), and to what degree it can be generally expected that a preservation action will preserve the different aspects of a digital object.

Although in some cases a given preservation methodology may lessen the degree of knowledge required in a given area, no preservation action will ever entirely eliminate the need to have at least some understanding in all these areas. For example, a preservation strategy that incorporates an emulation layer may not necessarily require a great deal of knowledge about the file structure, but may require more knowledge of the expected experience.

This document attempts to present each of the included preservation actions as valid ways of preserving digital materials, but also to help identify in which circumstances they may be more or less viable. In some instances, methodologies contain variations which may be more or less appropriate for different institution or individual circumstances, and for different types of collection materials. These have been identified in some instances.

For the purposes of this document, and in order to help facilitate the building of preservation strategies, the methodologies below are presented as facilitating either Primary Preservation Actions, or Secondary Preservation Actions. A Primary Preservation Action here indicates an action that directly changes the digital material/data. A Secondary Preservation Action here indicates an action that changes the way in which the digital materials/data is accessed (either to reproduce an experience or otherwise). For example, migration is seen as a Primary Action, as it directly changes the file itself in order to facilitate access. Alternatively, emulating an access environment for a file is seen as a Secondary Action, as it changes the way in which that file is accessed.

These actions have been presented from the perspective that many different methodologies may be employed to preserve a digital object over time, and in general more than a single preservation methodology will be required to adequately preserve any digital object. Indeed, it is proposed that it may generally be required to employ more than a single preservation action at once.Likewise, it is assumed that the preservation actions that it is possible to carry out on a digital object will vary and change over time, as will the most appropriate action. Initially, the appropriateness of preservation actions will depend upon the type of material and the intention of the individual carrying out the action. Further to this, however, the right actions to pursue may additionally be affected by the age or obsolescence of the material, which may initially rule out certain actions entirely. It may also change as knowledge about an object increases, or the preservation intent towards the object changes over time. In some cases, certain preservation actions may make it easier or more difficult to recover from these possibilities, and so this should be taken into account. Similarly, certain preservation actions may be required as pre-requisites for other actions, or in some instances, as vital parts of an institution or individual’s overall preservation strategy.

For these reasons, the actions described below are not presented as singular preservation paths, but as different elements that could contribute to the overall goal decided upon by the individual or organisation. In all cases, this document attempts only to provide an indication of our current thinking in these areas, and of the consequences or benefits of any given action, and should be taken as such.

This document was created to address a need at the NLA to try to understand and articulate to a broader audience the appropriateness of certain preservation methodologies. It is also intended for this document to be useful for facilitating a better informed conversation between business areas across the Library.

Primary Preservation Actions

Primary Preservation Actions are action which directly impact on the digital material/data being preserved. The methodologies presented below are ways in which this can be achieved.1.1 Migration 9 1.2 Take No Action 20